Background: While observational studies have established a connection between attention deficit hyperactivity disorder (ADHD), autism spectrum disorder (ASD), and heightened risk for cardiovascular diseases (CVD), the causal relationships are not well-defined. This study is designed to examine the causality between ASD, ADHD, and CVD risk as well as investigate the mediating factors through which ADHD and ASD influence CVD. Methods and Results: Leveraging two-sample Mendelian randomization (MR) approaches and large scale GWAS summary stats, we examined underlying causal links between ASD and ADHD and the risk of CVDs. The analysis indicated that ADHD was related to an increased likelihood of developing coronary heart disease (OR [95% CI] 1.12 [1.03, 1.21], p = 0.008), heart failure (OR [95% CI] 1.14 [1.07, 1.22], p = 1.45 × 10-4), and large-artery stroke (OR [95% CI] 1.35 [1.09, 1.66], p = 0.005). In parallel, ASD showed a correlation with a greater atrial fibrillation risk (OR [95% CI] 1.09 [1.03, 1.16], p = 0.005] and heart failure (OR [95% CI] 1.11 [1.04, 1.19], p = 0.004). Additionally, we explored the mediating role of CVD risk factors through two-step MR and multivariable MR, highlighting the possible role of smoking, prescription opioid use, triglycerides, education, income, Townsend deprivation index, and obesity in the causal association of ADHD, ASD, on CVDs. Conclusion: This MR study highlights the necessity for rigorous cardiovascular surveillance and interventions to decrease adverse cardiovascular events in people with ADHD or ASD by preventing identified mediating risk factors.